1. Field of the Invention
The present invention relates to a touch panel, and more particularly relates to a technique for improving the operability of a touch panel in environments that involve relatively low temperatures.
2. Description of the Related Art
A typical touch panel is constructed of an ultrathin glass plate (base sheet member) and a flexible transparent resin film (top sheet member) that are set facing each other and have resistive membranes on their opposed surfaces, with spacers being placed between the two sheet members in order to provide a certain spacing. When a user presses the surface of the transparent resin film using a stylus or the like, the two resistive membranes face each other at a certain distance come into contact at the pressed position. This changes the resistance between electrodes connected to each resistive membrane. Therefore, by detecting the resistance, information for the position pressed by the user can be obtained.
Touch panels are being used in an ever-widening range of applications as input terminals. As a result, additional features are also desired for the touch panels. As an example, when a touch panel is provided over a liquid crystal display (LCD) panel of a device that is used outdoors, such as a car navigation system or a portable computer, a touch panel may be glare-resistant by covering the panel with a polarizing plate. This stops the visibility of the display of the LCD panel from decreasing due to the reflection of incident light.
The polarizing plate used here is usually made by processing a resin film. Such a polarizing resin film has a thermal linear expansion coefficient eight to nine times that of the glass which forms the base sheet member.
A large difference between the thermal linear expansion coefficients of the top and base sheet members causes the following problems. As the temperature rises, the whole top sheet member including the polarizing plate bulges outward, ruining the appearance of the touch panel. The operability of the touch panel also decreases, as the space between the top and base sheet members widens and forces the user to apply a greater pressure to operate the touch panel. Due to the recent increases in the size of LCDs, the size of touch panels is also on the increase, so that outward bulge of the top sheet member is likely to increase, making touch panels even more difficult to operate.
In view of the above problems, the applicant of the present invention has devised a touch panel where the upper surface of the polarizing plate is covered with a resin film having a lower thermal linear expansion coefficient than the polarizing plate to keep the polarizing plate from bulging outward. An experiment was conducted on the operability of the touch panel in which a PET (polyethylene terephthalate) film is used as the bulge-resistant film. The bulge-resistant film was found to effectively prevent the polarizing plate from bulging outward with a rise in ambient temperature, and the touch panel demonstrated high operability.
On the contrary, even with the structure described above, the operability of the touch panel deteriorated when the ambient temperature fell below 0xc2x0 C. With the current demand for lighter portable devices, such as a portable computer, equipped with a touch panel, a thinner glass should be used as the base sheet of the touch panel. The use of a thinner glass would however further impair the operability of the touch panel at a low ambient temperature.
Portable devices equipped with a touch panel can easily be carried anywhere, and so may be used in a great variety of conditions. If taken outdoors in cold districts, devices equipped with a touch panel may end up being operated at an ambient temperature below 0xc2x0 C. Therefore, it is desirable to ensure high operability of the touch panel even in low temperature environments.
These problems that occur at low temperatures when the base sheet member is made thinner also occur when the top sheet member, and not just a polarizing plate, is made of a resin film with a high thermal linear expansion coefficient.
In view of the above problems, the present invention aims to provide a touch panel that does not suffer from poor operability at low temperatures, even if the base sheet member is made thinner.
The above object can be achieved by a resistive-membrane touch panel comprising: a first sheet member having a first resistive membrane on a main surface thereof; a second sheet member that is flexible and has a second resistive membrane on a main surface thereof, the second sheet member being opposed to the first sheet member with a spacer in between so that the second resistive membrane and the first resistive membrane face each other with a certain spacing therebetween; and a third sheet member that is provided on at least a part of another main surface of the first sheet member that does not face the second sheet member, wherein the third sheet member has a higher thermal linear expansion coefficient than the first sheet member.
With this construction, even when the second sheet member contracts by a greater amount than the first sheet member due to a low ambient temperature, causing the first sheet member to bend outward, the third sheet member with a higher thermal linear expansion coefficient that is at least partly laminated on the outer main surface of the first sheet member acts to suppress the outward bend of the first sheet member from outside. This prevents the distance between the two resistive membranes formed on the opposed surfaces of the first sheet member and the second sheet member from widening, thereby maintaining high operability of the touch panel.
Here, the second sheet member of the resistive-membrane touch panel may include a polarizing plate whose thermal linear expansion coefficient is higher than a thermal linear expansion coefficient of the first sheet member, and the thermal linear expansion coefficient of the third sheet member may be almost the same as the thermal linear expansion coefficient of the polarizing plate.
With this construction, the third sheet member contracts by almost the same amount as the second sheet member at low temperatures. As the above two sheet members contract in the same degree, the forces acted upon both sides of the first sheet member are well balanced, thereby the outward bend of the first sheet member can be suppressed by the proper pressure from the third sheet member.